Thermal overload relay with improved response

Abstract

Improved response in an ambient temperature compensated thermal overload relay is obtained by providing a bimetal element (2) wherein the main bimetal section (4) is coupled through a reversed compensating bimetal section (6) to a heat sink mounting base (8) either directly or through a heat transfer control member (18) and placing the heater (10) that is energized by the overload current only adjacent the main bimetal section (4). With this arrangement, the main bimetal section (4) bends in the switch tripping direction and the compensating bimetal section (6) bends in the subtractive direction in response to temperature change therein to reduce the net deflection for ambient temperature compensation under normal operating conditions and gradual small overload conditions. The heater (10) provides a temperature differential between the main (4) and reverse (6) bimetal sections although the reverse bimetal section (6) is subject to some conduction temperature rise above ambient temperature due to controlled thermal conduction from the main bimetal section (4) thereto and therethrough to the heat sink (8). But under fast high overload conditions, the reverse bimetal section (6) has insufficient time to incur a substantial amount of the conduction temperature rise to afford quicker tripping of the switch (16) for improved response. The ratio of the lengths of the main and reverse bimetal sections (4,6) is set or selected for the desired compensation.

We claim: 
     
       1. A thermal overload relay with improved response for monitoring an electrical system current comprising: a heat sink mounting member;   a thermal member supported at one end in cantilever manner on said heat sink mounting member so as to have a supported end and a free end;   said thermal member comprising a main bimetal section at its free end and a reverse bimetal section at its supported end coupled in heat transfer relationship with respect to one another and said heat sink mounting member and arranged so that said main bimetal section bends in the tripping direction and said reverse bimetal section bends in the substractive direction in response to temperature change therein to reduce the net deflection for ambient temperature compensation under normal operating conditions and gradual small overload conditions;   switch means effective when tripped for protecting said electrical system from overload current;   a heater in proximity to said main bimetal section responsive to said system current for heating said main bimetal section so as to provide a temperature differential between said main and reverse bimetal sections although said reverse bimetal section is subject to some conduction temperature rise above ambient temperature due to thermal conduction from said main bimetal section thereinto and therethrough to said heat sink mounting member;   and means coupling the free end of said main bimetal section to said switch means so that when said system current exceeds a predetermined value on said gradual small overload conditions said net deflection will be of sufficient magnitude to trip said switch means and when said system current rises faster to an abnormally high value so as to exceed said predetermined value by a large amount said reverse bimetal section has insufficient time to incur a substantial amount of said conduction temperature rise and consequent subtractive deflection due to said heat sink thereby providing a net deflection greater than said gradual overload net deflection to cause a quicker trip of said switch means to provide an improved response time for abnormally high overloads.   
     
     
       2. The thermal overload relay with improved response as claimed in claim 1, wherein: said main and reverse bimetal sections are bimetal strips of different lengths;   and said thermal member also comprises means for rigidly connecting said main and reverse bimetal strips to one another at their adjacent ends.   
     
     
       3. The thermal overload relay with improved response as claimed in claim 2, wherein: said means for rigidly connecting said main and reverse bimetal strips to one another comprises means for securing their adjacent end portions together in overlapped relation.   
     
     
       4. The thermal overload relay with improved response as claimed in claim 2, wherein: said reverse bimetal strip is shorter than said main bimetal strip.   
     
     
       5. The thermal overload relay with improved response as claimed in claim 2, wherein: said means for rigidly connecting said main and reverse bimetal strips to one another at their adjacent ends comprises means for connecting the same in predetermined heat conducting relation.   
     
     
       6. The thermal overload relay with improved response as claimed in claim 5, wherein: said means for connecting the same in predetermined heat conducting relation comprises means interposing a heat conduction control member between said adjacent ends of said bimetal strips.   
     
     
       7. The thermal overload relay with improved response as claimed in claim 6, wherein: said heat conduction control member is a sheet of mica between said adjacent ends of said bimetal strips that limits the rate of heat flow from said main bimetal section to said reverse bimetal section.   
     
     
       8. The thermal overload relay with improved response as claimed in claim 6, wherein: said heat conduction control member is a fiberglass mat such as a glassmat between said adjacent ends of said bimetal strips that limits the rate of heat flow from said main bimetal section to said reverse bimetal section.